The invention relates to echo cancellation and suppression in voice communication systems and more particularly a new echo canceling device wherein there is an adaptive suppression of the residual echo.
There are two kinds of echoes which may arise in a voice communication system: electrical echoes and acoustical echoes. The electrical echoes are due to impedance mismatch at the devices called  less than  less than hybrids greater than  greater than  where two-wire lines become four-wire lines. Hybrids are traditionally used when the two-wire local subscriber loop becomes a four-wire long distance trunk. In the telephony assisted by computer, the hybrid is also used at the point where the four wires (two wires from the microphone, and two wires from the loudspeaker) are converted into the two-wire local loop. The acoustic echo arises in a case when there is an acoustic path between the loudspeaker and the microphone so that part of the signal reproduced by the loudspeaker is reflected (weakened, delayed and distorted) by the room and then recorded by the microphone which is the typical situation when using a speakerphone.
Echo suppression is used to identify pure echo intervals in the primary signal which is a mixture of the signal received from the communication system and the echo signal, and suppress the echo signal therefrom. This method is usually effective, but it makes the conversation sound unnatural because of  less than  less than switching greater than  greater than  from an open channel (when primary side is talking to close channel and when primary signal consists mainly of echo) and vice versa. In double-talk situations (where both parties talk at the same time), the switching artifacts are very disturbing.
Echo cancellation is used to remove the echo part from the primary signal usually by means of adaptive filtering. In real environment, with non-linear effects and noticeable background noise, the echo is not perfectly canceled. Moreover, every change in the echo path (e.g. increasing the speakers volume or moving the microphone) will result in a loud echo until the system converges to the new path. The residual echo, though much less disturbing than the original one, can still be very annoying in some situations.
Suppression of residual echo left after echo cancellation is described in U.S. Pat. No. 5,563,944. When the residual primary level is small compared to the reference signal level, it is assumed to be echo and it is further suppressed by a predefined gain (in some systems background noise level is taken into account). The disadvantages of the existing methods are as follows:
1) The residual primary level can be high when the echo canceller does not perform well (e.g. when the canceller adapts to a new echo path). When this happens, the residual primary signal is not suppressed, even though it contains a strong echo.
2) When the system does suppress the residual echo, it usually uses a fix gain. Often the echo canceller performs well, and there is no need for strong suppression, which causes switching artifacts typical to echo suppression systems.
Accordingly, an object of the invention is to provide an echo canceling device wherein the residual echo is suppressed only by the amount needed to make it unnoticeable and thus avoiding switching artifacts.
The invention relates to an echo canceling device with adaptive suppression of residual echo in a voice communication system wherein a reference signal is sent over the system from a transmitting apparatus and comprising an echo canceller for removing any echo signal from a primary signal received by the transmitting apparatus, and a residual echo suppressor for removing the residual echo at the output of the echo canceller. Such a device is characterized in that the residual echo suppressor comprises means for defining speech echo intervals during which the primary signal contains mainly echo of the reference signal, first means for computing an estimation of the residual echo level, second means for computing a target level of residual echo, and third means for computing a suppression gain G to be applied during the speech echo intervals to the residual echo signal obtained after the application of the echo canceller, the suppression gain being the ratio between the target residual echo and the residual echo level estimation.